Many mollusks including snails and slugs, terrestrial as well as aquatic cause serious economic and health problems in many parts of the world. Snails which are members of a large class of gastropod mollusks including most forms having a univalve shell or having no shell can be quite injurious to vegetation as they destroy many varieties of beneficial agricultural plants. Even more harmful is the role that they play in the life cycle of many tropical and semitropical diseases. Millions of people and countless animals in many parts of the world are afflicted with these diseases. Snails play a significant role in the growth cycle of the parasite involved in these diseases. In the snails the parasite larval stages develop and emerge to enter warm-blooded animals and mature into worms. The worms in turn lay eggs which are carried to vital organs in the animal or human body by the bloodstream. Lastly, the eggs find their way back to the snails through water supplies and the like and the cycle begins once again. Thus, a single snail can be the ancestor of many millions of new snails per year.
For example, snails of the genre Oncomelania, Australorbis and Bulinus are schistosome intermediate hosts. Likewise, snails of the genre Lymnaea are intermediate hosts for the liver fluke worm. Snails of these genre particularly cause debilitating human problems. Specifically, bilharziasis has long been endemic in various parts of the world, and is even on the increase.
While various controls methods of bilharziasis and other diseases of this type have been suggested, the destruction of intermediate snail hosts by toxic chemicals appears to be the most rapid and effective means for reducing transmission of many tropical and semitropical diseases.
However, many chemicals useful in combating mollusks such as snails, such chemicals generically termed as molluscicides, have certain disadvantages. In some cases they are difficult to formulate and in certain types of habitats available formulations do not disperse effectively. In other instances the chemical itself is irritating and potentially dangerous to the handler, is required for use at relatively high dosages, and may be prematurely used up by absorption by soil and organic material. Again, other molluscicides on the market are ineffective at a high pH, are corrosive to equipment or their activity is reduced by bright sunlight. Lastly, some molluscicides while sufficiently active are inactivated at a low pH and/or do not kill snail eggs.
DDT [1,1,1,-trichloro-2-bis(p-chlorophenyl) ethane] has been widely used as an insecticide. However, its usefulness has diminished because of environmental hazards and a low degree of biodegradability.
Thus, continued use of DDT poses an environmental dilemma. Insecticides are required to control vector born diseases and to help protect the food supplies of the world. However, the micropollutants liberated into the environment by DDT threatens both the environment and the existence of many different animals. There is therefore, an urgent need to develop persistent, biodegradable insecticides that would act very much like DDT, but yet, would be rapidly biodegradable and then excreted.
DDT acts as an inducer of microsommal oxidase enzymes in the vertibrate liver. The injurious nature of DDT arises because of the stability of the aryl-chlorine bond, which is not attacked to any extent by the multifunction oxidase enzyme of living tissue. Therefore, compounds like DDT are stored in lipid tissues, instead of being metabolized and eliminated from the body. To produce biodegradable analogs, it is necessary to provide sites for attack by multifunctional oxidases.
It has been shown that by attaching "handles" to the DDT molecule, it is possible to obtain biodegradability. (Metcalf-Chemtech, Feb. 1972; 105) This biodegradability is produced by the fact that the multifunctional oxidases will attack the "handle", and thus, cause side chain oxidation to less lipid solubile molecules. As a result of these handles, biodegradability is obtained, and thus there is a much less toxic effect than with DDT. With these handles the multifunction oxidases, convert the lipid partitioning substrates of DDT into more water soluble molecules that are excreted from animal bodies, rather than being stored in fatty tissues and concentrated through ecological magnification.